In 1998 the Department of Housing and Urban Development commissioned a study by the National Association of Home Builders Research Center (NAHBRC) on housing trends. As part of that study the NAHBRC compared the cost of a house constructed on site, called “stick built,” to an identical house that was prefabricated, where prefabricated is defined as 98% of the house being factory built. The conclusion was that the construction cost of the prefabricated house was approximately 43% lower than an identical stick house. Given the relative difficulty of roof based labor, the savings for reducing roof based labor may be greater than the average savings on home construction. Thus, prefabrication is an important field for installed solar array system cost reductions.
In addition to installing solar array systems on prefabricated structures, solar array systems are installed on sites such as existing residential and commercial buildings. A number of issues arise with the logistics of shipping a solar array system to sites. The prefabricated sections of the solar array system must be small enough and light enough to ship to the site and install without resulting in additional costs and labor. For example, common ladder based power conveyors will move approximately 350 pounds. Therefore, a solar array system weighing less than 350 pounds would be desirable because the solar array system would be capable of being transported by a common ladder based power conveyor. Furthermore, even if the solar array system is within an acceptable weight range, the size of the solar system array must be manageable for laborers working on a roof. The ability to fold the array into a reasonable size also affects transportation to the work site by common carrier. For example, local installers may use pick-up trucks or small box trucks for local delivery. Therefore, having a smaller package size that is compatible with vehicles of this size may enhance results.
Solar arrays consist of a set of solar panels. Solar panels are a type of photovoltaic cell. A photovoltaic cell is a semiconductor device that converts the energy of light into electricity using the photovoltaic effect. When light shines on a photovoltaic cell, a voltage develops across the cell. The voltage results in a current when a load is electrically connected to the photovoltaic cell. The voltage and current vary according to environmental factors (e.g., the amount of light shown on the photovoltaic cell, the temperature of the photovoltaic cell) and design characteristics (e.g., dimensions of the photovoltaic cells, materials to used to construct the photovoltaic cell) of the photovoltaic cell.
To get the maximum power from the solar array system, an inverter associated With the solar arrays typically employs maximum power point tracking (MPPT). Individual solar panels have a complex relationship with other panels in the solar array system due to variations in the environmental factors and design characteristics. The differences in the individual solar panels can drag down the efficiency of the entire solar array system. In fact, when connected in a string configuration (e.g., series connection) the output of each solar panel is limited to the output of the lowest performing solar panel in the string. The output of the array of N panels is then N times the output of the lowest performing panel.
Furthermore, the individual panels are typically Wired to an inverter using direct current (DC) cabling. DC cables are used in the field because photovoltaic cells are semiconductor devices and semiconductor devices are DC devices. To outfit the solar array system in the field would require additional materials (e.g., devices, cabling) and require additional time and labor. Furthermore, it is difficult to install the individual elements in the field because the individual elements must be individually shipped, amassed onsite (e.g., rooftop), and installed on site. The individual elements may need to be individually tested to ensure that they operate correctly with the system. There are a number of challenges to creating a solar system array that is practical from a shipping, installation, and setup perspective.